Polyalkyl methacrylates, polyisobutylene, hydrogenated products of styrene-isoprene copolymer rubber, etc. are currently known in the art to be useful as viscosity index improvers for lubricating oils. These lube oil additives work well in improving the viscosity indices of lube oils, but require the conjoint use of large amounts of detergency and dispersing agent and an antioxidant because for use in inbricating oils for internal combustion engines, such additives are required to have a high level of detergency and antioxidation property so as to prevent deposition of a varnish-like material, sludge and cake-like material on the engines.
Use of olefin copolymers as a lube oil additive is also known. However, the use of high-molecular-weight olefin copolymers in this field of application has the defect that the shear stability of a lubricating oil containing this additive is very poor although an effect of increasing its viscosity is noted.
Because a low-molecular-weight olefin copolymer is difficult to produce commercially, it has generally been suggested to degradate an easily producible high-molecular-weight copolymer to a low-molecular-weight product and to use it as an additive for lubricating oils. To reduce the molecular weight of olefin copolymers, methods comprising degradation with heat, oxygen, air, etc. are known (for example, U.S. Pat. Nos. 3,499,741 and 3,524,732).
Olefin copolymers whose molecular weights are reduced by these known methods have an unsaturated bond or oxygen-containing functional groups such as a hydroxyl, carbonyl or hydroperoxide group. Accordingly, when these copolymers are used as solutions in lubricating oils, they polymerize, decompose, or give excessively oxidized products. This consequently causes the disadvantage that a precipitate is formed, the viscosity of the lubricating oil is markedly increased or at times reduced to render the viscosity unstable. In actual use, therefore, large amounts of a detergency and dispersing agent and an antioxidant must be used together.
On the other hand, when olefin copolymers are used as additives for lube oils, they are still not entirely satisfactory with regard to detergency and dispersing property and oxidation stability although this is not to such an extent as the aforesaid degradation products of olefin copolymers. For example, when an oxidation stability test based on JIS K-2514 is conducted, a great amount of deposit is seen to adhere to a varnish stick. When a panel coking test, which is a coking test well known in the art, is performed, the amount of sludge formed is large. In actual use, therefore, it is necessary to use a detergency and dispersing agent, an antioxidant, etc. conjointly.
For this reason, the development of a multifunctional viscosity index improvers having detergency and dispersing property or an oxidation inhibiting ability and a lubricating oil composition comprising it has been desired in the market, and there has been a strong interest in it.
Thus, improvements have been suggested, for example a method which comprises grafting an oxygen-containing monomer to an olefin copolymer whose molecular weight has been reduced by means of heat and/or oxygen (Japanese Patent Publications Nos. 19112/74 and 28288/76; corresponding to British Pat. No. 1,244,432), a method which comprises grafting a polar monomer having a vinylidene group to the aforesaid copolymer (Japanese Patent Publication No. 42685/73; corresponding to French Pat. No. 68156134), and a method which comprises grafting a nitrogen-containing polar compound to the aforesaid copolymer (Japanese Laid-Open Pat. Publication No. 18705/77).
However, since these known methods require a two-step modifying procedure consisting of the oxidation of the copolymer rubber and then the graft copolymerization, the operation of obtaining a lube oil additive is complicated. Moreover, the modifying reactions have poor reproducibility. Furthermore, in the oxidation stage, a hydroperoxide group or a carbon-carbon double bond, which becomes a cause of degradation, forms on the polymer chain. Thus, in the subsequent grafting reaction, the oxidation product tends to undergo crosslinking and degeneration. Accordingly, the aforesaid properties do not reach entirely satisfactory levels.
As an additive intended for improving the viscosity increasing effect of lube oils, there is also known a viscosity index improver obtained by copolymerizing a vinyl aromatic compound with an acrylate and/or a methacrylate in the presence of an oil-soluble hydrogenated product of a copolymer of a vinyl aromatic compound and a conjugated diolefin (Japanese Laid-Open Pat. Publication No. 39305/78). This viscosity index improver differs from the present invention in that the proportion of the copolymer of vinyl aromatic compound and a conjugated diolefin is low, and the monomers to be post-polymerized (grafted) are not only the acrylate and/or methacrylate but also the vinyl aromatic compound. The viscosity inded improver of this suggestion still leaves room for improvement in viscosity increasing effect and detergency and dispersing property.
As described in detail hereinbelow, the present invention relates to a multi-functional lubricant oil additive, a lubricating oil composition containing it, and to a process for preparation thereof which are different from the conventional compositions and the methods for production thereof.